Net Positive Suction Head ( NPSH ) is extremely important to the operation of a centrifugal pump. Factors that affect NPSH are atmospheric pressure, suction line friction loss, elevation, fluid temperature and SG.
Hoppers, mud guns, desanders, desilters, degassers, and triplex pumps requiring supercharging all have one thing in common: they require 76–80 feet of inlet head to operate as designed. Exceptions do exist, and the equipment manufacturer should be consulted. This simplifies the job of sizing centrifugal pumps. Since most applications in drilling systems require 80 feet of head at the inlet of the equipment, knowledge of volume needed by each piece of equipment is required. Following are standard flow rates when equipment has an 80-foot inlet head:
Turbulent flow is detrimental to a centrifugal pump during handling of abrasive fluids. The drilling industry has standardized centrifugal pumps with concentric casings and wide impellers, a design that has proven to offer less turbulence and greatest pump life. The walls of a concentric style of casing (Figure 1) are an equal distance from the impeller throughout the impeller circumference, resulting in a smooth flow pattern. A volute style of casing (Figure 2) has a cutwater point that disturbs the fluid flow pattern, creating an eddy.
Standard centrifugal pumps are not self-priming and require the fluid end to be primed prior to activation. This can be accomplished by installing the pump in a location that provides a flooded suction or by using a device to prime the pump. Once the pump casing is full of fluid, it can then be energized. Running a pump dry or restricting suction flow can severely damage the fluid end, mechanical seal, or packing. The designs of self-priming pumps result in turbulent flow patterns, which cause excessive wear during pumping of abrasive fluids and increase operating costs. The drilling industry avoids using self-priming pumps due to increased downtime and costs.